1. Field of the Invention
The invention relates to a roundsling which has an inner non-interwoven structure and a covering which receives the inner non-interwoven structure, the covering taking the form of a hollow braided structure and its end portions being inserted in one another and being connected together to overlap one another in a connection zone.
Roundslings of this kind are used to lift loads. For this purpose, the roundsling is first placed around the load to be lifted in such a way that the load rests in the opening in the sling, and the roundsling then hooked onto the hook of a crane or a comparable piece of lifting equipment.
A roundsling usually consists of a wound loop of yarn (the inner non-interwoven structure) which carries the load, which is covered by a protective covering comprising a braided structure. The inner non-interwoven structure may equally well be formed by a wound loop of wire rope or stranded cable.
The protective covering of the roundsling protects the non-interwoven structure against damage caused by wear or cutting loads and also groups the many strands of the inner non-interwoven structure together into a single easily handable strand.
2. Description of the Related Art
There are various possible ways in which conventional roundslings can be manufactured. One method of manufacture which has become widely used in practice is described in, for example, DE 37 03 547 A1 and in this method a woven tube is produced beforehand and is positioned on a roundsling machine when upset in its longitudinal direction.
In the winding which follows, the individual loops of yarn forming the inner non-interwoven structure are threaded into the tube until the inner non-interwoven structure reaches an adequate thickness. The sling which is obtained in this way is taken out of the machine and the ends of the tube are sewn firmly together. This is followed by the finishing.
A prerequisite for manufacture in the manner explained above is a high ratio between the cross-sections of the non-interwoven structure and the covering. This is the only way in which the covering can be upset and the wound loop forming the inner non-interwoven structure produced with the requisite reliability. A consequence which the necessarily large cross-section of the covering has is that the finished roundsling has a relatively flat cross-sectional shape and the inner non-interwoven structure is held relatively loosely in the covering.
It is true that the clear cross-section of the covering can be reduced at a later stage, and the inner non-interwoven structure can be held more tightly in the covering, by sewing in a ridge-like pleat which is left in place at the side. However, it is still unavoidable that the known roundsling will be of a flat shape. This results in considerable disadvantages in practical use.
Because the roundsling necessarily follows radiuses at the surfaces which rest against it, the woven structure of the flat covering inevitably forms creases at the bottom end of the roundsling which is associated with the load. These creases cause increased wear because the covering is subject to a greater chafing load in the region of the creases than in the region of the remainder of its outside surface. The creasing also leads to a reduction in the maximum tensile strength of the roundsling, because the thickenings which occur interfere with the transmission of the load in the inner non-interwoven structure which carries the load. A complicating factor in this case is that the individual yarns of the inner non-interwoven structure, which latter rests relatively loosely in the protective covering, are able to move relative to one another and as a result the yarns of the non-interwoven structure are not, as a whole, evenly loaded. Also, the loose make-up of the roundsling and the generous dimensions of the covering when compared with the diameter which the inner non-interwoven structure assumes make handling more difficult as a result of what could be said to be the excess amount of covering material, because the sling is floppy or slack and as such is for example difficult to fit over a hook or a comparable projection.
Improved properties in use and a longer life due to an appreciably reduced proneness to wear are exhibited by roundslings whose covering takes the form of a hollow braided structure. A roundsling of this kind is described in DE 10 2006 052 279 A1 for example.
In roundslings of this kind, use is made of the possibility which is known per se from the field of conventional core-and-sheath cables of surrounding an interior yarn core with a braided covering. The hollow braided structure can be braided directly around the inner non-interwoven structure in this case. Alternatively, it may be produced beforehand to then be filled with the inner non-interwoven structure.
In the latter case, a hollow braided structure has the advantage that it can be upset with an accompanying enlargement of its clear cross-section and becomes narrower again when it is then pulled straight. The upsetting involves, at the same time, a shortening of the length of the hollow braided covering. In exactly the same way, stretching out of the hollow braided structure results not only in a reduction in the diameter of the covering but also in a lengthening of the actively effective length of the covering.
Basically, it is possible for the ends of a pre-produced hollow braided covering to be connected together in any suitable manner in such a way that any unintentional disconnection of the covering is reliably prevented and that there is assured protection for the inner non-interwoven structure at all times in use. For this purpose, the ends of the covering may for example be adhesive-bonded, welded or sewn together.
However, it practice it has been found that there are disadvantages to every one of these possible methods of connection. In this way, adhesive-bonded or welded connections are found to be a problem if the covering has to carry certain tensile loads while items are being moved. In this case the adhesive-bonded or welded connection is subject to adverse shear forces which may result in the connection peeling apart or tearing. The sewing together of the ends of a hollow braided covering has the disadvantage that the covering, which rests tightly against the inner non-interwoven structure, cannot easily be sewn together with conventional sewing machines in such a way that the seams will withstand even high loads. Added to this is the fact that the sewing together of the covering can often only be performed in such a way that the inner non-interwoven structure is pierced by the yarn which makes the seam. This may result in the individual strands of the non-interwoven structure no longer being able to align themselves in the optimum way under load, which may cause premature fracture particularly in the case of fibres which, although they may have a high carrying capacity, are at the same time only able to stretch to a small extent.
DE 20 2007 001 484 U1 shows a cable sling comprising a hollow braided structure which is braided without a core, the ends of this cable being connected together by splicing, for which purpose openings are provided in the ends of the cable.